A common problem faced by authors of confidential documents is the difficulty of preventing unauthorised persons from gaining access to them. A related problem is breach of copyright of printed texts. These problems have become particularly acute with the commonplace availability of photocopiers as it is a simple matter for a person legitimately possessing a certain document to copy it and pass it on to other, perhaps unauthorised, persons. To date, prevention of such transgressions has been mainly by physical restriction of availability, for example by denying access to all but a trustworthy few, and relying on punitive measures to prevent breach of confidence. As a further deterrent confidential documents are sometimes stamped with the words "SECRET" or "CONFIDENTIAL". Such techniques have little effect and efforts have been made to mark documents in such a way that each copy bears a distinctive feature so that in the event of "leaking" the source of the leak can be identified. One such technique involves subtly changing the text of each copy distributed and recording the details of changes made in the case of each recipient. This technique is cumbersome, requires undesirable alteration of the text, and relies on recovery of the whole or a large part of the text to be effective as a means of identifying the source. Other marking schemes utilise a pattern of special markings which must cover the whole page area if the possibility of erasure is to be avoided.
The present invention is directed to providing an improved document marking scheme which uniquely identifies each of a potentially large number of copies of a document and cannot be removed from the text of the document. A further novel and useful feature of the invention is the ability to convey within the marking textual information, such as the name of the intended recipient of each copy. Furthermore the marking scheme of the invention does not significantly detract from the visual quality of the document, and the marking is maintained in spite of photocopying, enlarging or reducing. The invention can be realised by a suitable arrangement of conventional digital data processing components. Other advantages and objects of this invention will be more fully appreciated from the description of the invention which follows.